1. Field of the Invention
The present invention relates to a continuous process for producing expanded beads of a thermoplastic resin and the apparatus used in the continuous process. More particularly, the present invention pertains to a process by which an expanded bead is obtained by continuously pumping a slurry of thermoplastic minipellets containing a blowing agent through a series of impregnation vessels at a predetermined pressure and temperature, and at a sufficient impregnation time to allow the blowing agent to impregnate the thermoplastic particles, and subsequently releasing the slurry into a low pressure expansion tank. Instead of preparing expanded thermoplastic beads batchwise, the present invention continuously feeds the raw materials into a slurry tank for mixing and agitation, following which the slurry is subsequently pumped through heat exchanger(s) and impregnation vessel(s) to impregnate the blowing agent into the thermoplastic minipellets. Unlike the batchwise process, the continuous process need not employ an inert gas filled vapor space to force the blowing agent into the minipellets during the impregnation phase.
2. Description of the Related Art
Prior processes for the manufacture of expandable thermoplastic minipellets revolved strictly around batchwise processes. A typical batchwise process for expanding polypropylene (PP) beads proceeds as follows. A jacketed high pressure vessel such as an autoclave with a typical 400 gallon capacity is charged with thermoplastic beads, water, and a dispersant at atmospheric pressure, and the ingredients are thoroughly mixed by agitation. The raw materials only partially fill the autoclave, leaving a void above the mixture. The autoclave is then closed and the void is purged of any oxygen by pumping an inert gas such as nitrogen through the vessel. Subsequently, a blowing agent such as butane is charged into the autoclave at about room temperature and at a pressure of about 90 p.s.i.g. This pressure and temperature are close to the vapor pressure of butane. Therefore, a slight amount of nitrogen, or inert gas, is charged into the vapor space within the autoclave at 110.degree. F. to 125.degree. F. to raise the pressure inside the autoclave to about 110 p.s.i.g., thus driving the butane into the liquid phase. The charging process of all elements commonly takes about 30 minutes. While agitating the thermoplastic/butane dispersion, the autoclave is steam heated usually above the softening point of the thermoplastic particles for up to 50-60 minutes, or enough impregnation time to ensure that the thermoplastic minipellets are impregnated with the blowing agent. Pressures inside the closed autoclave often rise to 300-400 p.s.i. during the heating process, which is much greater than the vapor pressure of the blowing agent. When the PP beads are sufficiently impregnated with the blowing agent, an inert gas such as nitrogen is released into the vapor space in the autoclave at a high pressure ranging from 400-450 p.s.i. The high pressure nitrogen in the vapor space serves to provide the necessary force to eject all the PP beads out of the autoclave at a pressure high enough to prevent most of the blowing agent from escaping out of the PP beads while in the autoclave. Without the high pressure nitrogen in the vapor space above the slurry, the pressure inside the autoclave decreases as the minipellets discharge, the latter discharged beads experiencing a smaller pressure drop across the orifice ultimately resulting in a wide variance in the degree of expansion in the beads. Once the high pressure nitrogen is charged into the vapor space above the slurry in the autoclave, the contents are evacuated by opening a valve from beneath allowing for a rapid release of the aqueous dispersion through an orifice from a high pressure state within the autoclave to a low pressure state in the evacuation tank. The large sudden pressure drop across the orifice upon release of the dispersion into the evacuation tank causes the blowing agent trapped in the thermoplastic particle to volatize, resulting in an expanded thermoplastic bead. The evacuation process typically takes only 90 seconds to complete. The bulk density of a typical bead at 900 g/l can be lowered to a 15 to 30 g/l expanded bead during the result of the expansion process which is a 30-60 fold volumetric increase. Such large volumetric increases require an evacuation tank considerably larger than the autoclave.
The water and partially dissolved dispersant evacuated along with the beads are separated out from the expanded thermoplastic beads, and the expanded beads are then dried with hot air. Due to the large volume of expanded beads produced at one time in a batchwise process, all equipment employed to receive, separate, and dry the beads must have large capacities to keep the cycle time to a minimum.
The nitrogen/blowing agent gases are released into the atmosphere or may be partially recovered, separated, and collected for future re-use. Various patents illustrating this typical batchwise process are U.S. Pat. Nos. 4,686,087 to Maeda et al; 4,689,351 to Endo et al; 4,602,047 to Endo et al; and 4,676,939 to Kuwabara et al.
There exist several inherent disadvantages in the batchwise process just described. A common cycle time runs about 90 minutes per batch, which introduces a significant production limitation. Equipment costs are high due to the necessity for an autoclave of a sufficient size to hold large batches of the dispersion, an even larger low pressure evacuation tank to accommodate the expanded beads, and large capacity transfer, dewatering, and drying equipment to process the large volume of expanded beads. Nitrogen and blowing agent are lost to the atmosphere, or if recovered, require significant expenditures in collectors/separators. In the batchwise process it is often necessary to prepare many batches for a large order, introducing the risk of deviations in the specified bead quality standard from batch to batch. Another significant drawback is that within a single batch, the butane to mini-pellet ratio is usually not uniformly distributed throughout the batch during the heating and impregnation phase, resulting in significant variations in expansion from bead to bead, often causing greater than a 25 percent drop in product quality. Furthermore, in spite of using high pressure inert gas in the vapor space above the slurry to quickly and forcibly eject the beads out of the autoclave, there still remains enough of a pressure drop within the autoclave to significantly impact the uniformity among the beads. This problem has been partially alleviated by continuously pumping a high pressure inert gas into the autoclave during the release of the beads to maintain uniform pressure, but has not been completely solved by this approach. This solution adds extra equipment costs and does not address the problem of the non-uniform butane to pellet ratio during the heating and impregnation phase.